https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117214
Bug ID: 117214
Summary: chrono format error for time point using %c: D_T_FMT
cannot be directly used as chrono-specs
Product: gcc
Version: 14.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: xu2k3l4 at outlook dot com
Target Milestone: ---
test code:
```cpp
#include <chrono>
#include <locale>
#include <print>
int main(int argc, char **argv)
{
auto timepoint = std::chrono::sys_seconds();
for (int i = 1; i < argc; ++i)
{
const char *localename = argv[i];
std::locale::global(std::locale(localename));
try
{
std::println("{}: {:L%c}", localename, timepoint);
}
catch (std::format_error &e)
{
std::println(stderr, "ERROR {}: {}", localename,
e.what());
}
}
}
```
`locale-gen` many locales, and run the program with these locales. The program
shows 4 kinds of outputs (x86_64-pc-linux-gnu gcc 14.2.1):
1. "(some normal time point)", which is good.
2. "chrono format error: invalid specifier in chrono-specs", e.g. aa_DJ (uses
%l), ar_SA (uses %k), ca_AD (uses %-d).
3. "chrono format error: no '%' at start of chrono-specs", e.g. az_IR (starts
with U+202B <RIGHT-TO-LEFT EMBEDDING>).
4. "chrono format error: invalid modifier in chrono-specs", e.g. my_MM (uses
%OC).
#3 is easier to fix. C++ standard [time.format] says chrono-specs must starts
with a '%' (probably to seperate chrono-specs with fill-and-align & width &
precision). libstdc++ should output characters before any '%' and have the
sub-`vformat_to`'s format string start with '%' in `__formatter_chrono::_M_c`.
#2 and #4 are somehow tougher. chrono-specs follows strftime's format, but
D_T_FMT from glibc contains glibc extensions which are considered invalid by
chrono-specs. Maybe libstdc++ can directly use `strftime` in libc, or just
filter out these extension (e.g. substitude %l as %I, %k as %H, %-* as %*)?
